1. Field of the Invention
The invention relates to a linear actuator, and to a pump apparatus and compressor apparatus using a linear actuator.
2. Related Background Art
In the past, because even in cases of a pump or compressor in which a piston moves linearly within a cylinder, an actuator used therein made use of a motor with a rotational movement output, a crankshaft being provided between the output shaft of the motor and the piston rotational movement to linear movement, there existed the problem of low power transmission efficiency.
The inventor of the present invention considered a new type of linear actuator having, as shown in FIG. 6(A) of the accompanying drawings, a yoke structure 510 having a first yoke part 511 and a second yoke part 512 opposing the first yoke part 511 from a direction perpendicular to the axial direction thereof, a coil 560 generating an alternating magnetic field in the yoke structure 510, a magnet 530 disposed between the first yoke part 511 and the second yoke part 512, in which north and south poles are formed along the axial direction, intermediate yoke parts 521 and 522 disposed between the first yoke part 511 and the second yoke part 512 so as to surround the magnet 530 from both sides in the axial direction and so as to form a first gap 506 and a second gap 507 between itself and the first yoke part 511 and second yoke part 512, and a magnetic body 551 disposed so as to be movable in the axial direction relative to the gaps 506 and 507. In this arrangement, a gap 519 is provided between the second yoke part 512 and the intermediate yoke parts 521 and 522, so that a magnetic field is formed as indicated by either the arrow B31 or the arrow B32 in FIG. 6(A) and FIG. (B) respectively, as will be described below.
In an actuator as described above, a magnetic field is generated as shown by the solid-line arrows B1 and B2 in FIG. 6(A). In this condition, when an alternating current is caused to flow in the coil 560, during a period of time in which a current flows in the coil 560 in a direction from the drawing outward toward the viewer, a magnetic field indicated by the broken-line arrow B31 is generated and, of the gaps 506 and 507, whereas in the gap 506 at the first intermediate yoke part 521 side the orientations of the magnetic field from the magnet 530 and the magnetic lines of force from the coil 560 are mutually opposite, in the gap 507 at the second intermediate yoke part 522 side the orientations of the magnetic field from the magnet 530 and the magnetic lines of force from the coil 560 coincide, thereby resulting in a downward directed force acting on the magnetic body 551.
In contrast to the above, as shown in FIG. 6(B), during a period of time when a current flows in the coil 560 in the direction into the paper when viewing the drawing, a magnetic field indicated by the broken-line arrow B32 and is generated and whereas in the gap 506 at the first intermediate yoke 521 side, in which there is coincidence between the orientations of the magnetic field from the magnet 530 and the magnetic lines of force from the coil 560, in the gap 507 at the intermediate yoke 522 side, the orientations of the magnetic field from the magnet 530 and the magnetic lines of force from the coil 560 are mutually opposing, thereby resulting in an upwardly directed force acting on the magnetic body 551.
Because the orientation of the force applied in the axial direction to the magnetic body 551 alternates each time the polarity of the current flowing in the coil 560 reverses, an armature (not shown in the drawing) formed as one with the magnetic body 551 vibrates in the axial direction, making it possible to output reciprocating linear motion.
In a linear actuator as described above with reference to FIG. 6(A) and FIG. 6(B), however, because the formation of a magnetic field indicated by the arrow B31 or the arrow B32 necessitates the provision of a gap 519 between the second yoke part 512 and the first and second intermediate yoke parts 521 and 522, a considerable portion of the magnetomotive force obtained by the coil 560 is consumed in the gap 519, this contributing to a reduction in the magnetic efficiency and resulting in a small output. This has prevented practical application, and there remains a need for further improvement.
Accordingly, the present invention seeks to provide a linear actuator with a further improvement in output characteristics, in addition to a pump apparatus and a compressor apparatus using such a linear actuator.